1 2 Protoplasm : 



Cell Division. 



The manner in which the division of cells is brought about and 

 the dividing-wall built is wonderful in the extreme. It is preceded 

 by the division of the nucleus, and also, in animal cells by that of 

 the centrosome. The whole complex process has been studied in 

 the cells of numerous plants and animals, and is found to take 

 place exactly in the same manner in all of them. 



1. The nucleus first increases in size, the filmy wall surround- 

 ing it disappears, the ravelled thread opens its coils, and in animals 

 the centrosome divides in two, one of the halves migrating to the 

 opposite side of the developing nucleus. 



2. Secondly, the nuclear thread splits up into a certain number 

 of equal pieces called chromosomes, the number of which varies 

 considerably in different species, but remains constant in the same 

 one. These pieces shorten themselves and grow thicker, and the 

 coloured granules attached to them are thus brought into more 

 equal distance from each other. 



3. Thirdly, a number of fine threads suddenly make their ap- 

 pearance, which form a sort of spindle that in animal cells lies be- 

 tween the two centrosomes at the opposite sides of the nucleus. 

 The same kind of spindle is formed in plants, and though they 

 have no visible centrosomes, lies in the same direction as if such 

 were present. 



4. Fourthly, the chromosomes move towards the equator or 

 broadest part of the spindle, each along one of the spindle threads, 

 and as they go all of them divide lengthwise, together with all the 

 coloured granules along their length, so that each half of the threads 

 of the chromosomes receives an equal number of half-granules. 



5. In the fifth place, one half of each longitudinally-divided 

 chromosome, with its attached half-granules, separates from the 

 other, and two half-nuclei are formed consisting of the same number 

 of half-chromosomes. Each of these loops itself in the shape of 

 the letter V, and with their points turned towards the centrosomes, 

 the two sets proceed in opposite directions along the spindle 

 threads to the poles or narrow end of the spindle. There the 

 chromosomes of each half-nucleus unite again and form a continu- 

 ous thread which ravels itself up as in the original nucleus before 

 it prepared to divide. In this elaborate way two equal nuclei are 



